I just went through a few of these last week for a simple store.
getshopped.org, lots of themes and customization, lack of documentation, addon's will cost you. Definitely the most powerful out of the box
cart66, seems ok, limited functionality but supported. Few themes to make your cart look customized
shopp, alot of people complained about this one in general so I didn't even install it but support looks good, no out of the box themes.
marketpress, didn't bother because the premium version starts at $39 and goes up.
There were definitely some more out there but these 4 seemed to get the spotlight. Doing anything advanced will required php coding. I think for someone with a very small catalog they could get away with WP for ecommerce. I was checking out Magento and saw a number of WP plugin's interfacing into that platform, but that adds a whole new set of issues and complexity.